This invention relates to the continuous application of organic polymeric compositions to moving substrate surfaces to form a thin coating of the composition on the surface. If the coating remains on the substrate surface to which it is applied and is caused or allowed to harden or set, the process may be generically referred to as painting the substrate surface.
More particularly, but not exclusively, the invention is directed towards the continuous painting of moving metal, for example steel, strip in the context of an industrial production line for producing painted stock material in large quantities.
Probably the most commonly adopted mass production processes for painting moving strip comprise applying a thin layer of liquid paint composition, for example pigmented organic polymeric particles and filler particles dispersed or dissolved in a liquid solvent, to the strip, and causing or allowing the solvent to evaporate to leave an adherent, solid coating on the strip. The formation of the liquid layer on the strip may be effected in various ways, for example by dipping the strip into a bath of the paint composition followed by stripping surplus composition from the strip, spraying the paint composition onto the strip or by contacting the strip with a roller laden with the paint composition.
Such processes using solvent rich, low viscosity compositions are not entirely satisfactory. In particular, the solvents are dangerous if inhaled, expensive and environmentally damaging, thus it is essential that they be drawn off and condensed for re-use. This requires expensive equipment and otherwise unnecessary precautionary procedures that complicate the painting operation itself. Such processes do have the advantage that the tendency of the low viscosity liquid layer to flow before solidifying and the effect of surface tension tend to flatten the surface of the liquid layer resulting in an attractive smooth surface on the finished painted product.
Nevertheless the disadvantages of solvent based compositions are such that processes utilising substantially solvent free paint compositions, so called high solids compositions, have been developed wherein the composition has been rendered sufficiently fluid for it to be spread upon the substrate by controlled heating of the composition immediately prior to its application to the substrate. It has been found that, in general, application of such compositions to a substrate at a temperature of less than 20xc2x0 C. is inappropriate because the viscosity of the composition is too high. The extent to which the viscosity may be lowered by heating is limited because of the deleterious effect of high temperatures or prolonged heating on the characteristics of the paint composition. The upper temperature at which the compositions can be applied to the substrate is usually about 200xc2x0 C., although this temperature is somewhat dependent upon the particular composition being used. At these elevated temperatures, it has been found that excessive cross-linking can occur prior to paint film formation. This means that in processes using high solids compositions one cannot rely on the self leveling effect of a low viscosity liquid layer to produce a smooth surface on the finished coating to the same extent as one can in processes using solvent rich compositions. That problem is exacerbated if one takes full advantage of the accuracy with which the thickness of a paint coat may be determined when using high solids compositions to create thin coats (of the order of from 10 to 20 micrometers in thickness) as disclosed in, for example, the complete specification of Australian Patent No. 622948 (John Lysaght (Australia) Pty. Ltd. et al). Thin coats set quicker than thick ones and this further detracts from the self-leveling slumping of peaks in the surface of the applied coat.
Hitherto that disadvantage of using high solids compositions has been addressed by contacting an initial deposit of paint composition on the moving substrate surface with a so called smoothing roll (as disclosed in the mentioned Australian patent specification), which is positioned to make a near approach to the substrate surface to form a nip therewith, through which nip the deposit moves. This has the effect of spreading and smoothing the initial deposit. It may also limit the thickness of the deposit carried away from the nip on the substrate surface. That prior art procedure is referred to as xe2x80x9ca method of the kind describedxe2x80x9d hereinafter.
An object of the present invention is to provide a thin paint coat on a substrate surface, by a continuous coating procedure utilising high solids, organic polymeric paint compositions, having a smoother surface than has been attainable by such procedures hitherto. For the purposes of the invention a high solids composition may be defined as one having a so-called volume solids of at least 80 percent, preferably more than 95 percent. The term xe2x80x9cvolume solidsxe2x80x9d is a reference to the volume of the solids in the composition expressed as a percentage of the volume of the total composition.
The invention attains that object by the selection of parameters controlling the operation of methods of the kind described.
The invention consists in a method of providing a paint coat of an organic polymeric paint composition on a moving substrate surface, of the kind comprising establishing a quantity of the paint composition, in a high solids form at a temperature such that it is spreadable, in a nip defined by the substrate surface and a smoothing roll to enable paint from the established quantity to pass through the nip as a paint layer on the substrate surface, wherein the smoothing roll has a surface roughness coefficient (usually referred to by the symbol Ra) of no more than 1.5, and wherein the maximum surface speed of the smoothing roll expressed as a percentage of the substrate speed bears a linear relationship to the substrate speed, such that the surface speed of the smoothing roll is no more than 1.2 percent of the substrate speed at an exemplary low substrate speed of 15 meters per minute and no more than 12 percent of the substrate speed at an exemplary substrate speed of 150 meters per minute.
In preferred embodiments of the invention the Ra is no more than 0.8. The term surface roughness coefficient or Ra is a term of art defined, with reference to a sectional profile of a surface, as the arithmetic mean of the departure distances of the peaks and troughs of the profile from the mean line of the profile, expressed in micrometers. Thus the smaller is the value of Ra then the smoother is the surface.
For preference the direction of movement of the surface of the smoothing roll at the nip is the same as that of the substrate. It will be noted that the invention includes within its ambit the instance of a stationary smoothing roll having zero surface speed and instances wherein the direction of movement of the smoothing roll surface is opposite to that of the substrate.
By way of example two embodiments of the above-described invention are described in mole detail hereinafter with reference to the accompanying drawings.